Mechanical, morphological and comparative properties of microbeads assembled from carboxylated cellulose nanocrystals

While carboxylated cellulose nanocrystal (cCNC) microbeads are an emerging class of sustainable alternatives to microplastics, their expanded potential lies in diverse fields like drug delivery, cosmetics and personal care, agriculture, chromatography, water remediation and microencapsulation. Among the functional attributes relevant to these applications are the mechanical and morphological properties of cCNC-derived microbeads, where the cCNC nanorods are quench-condensed into spheres by spray drying. Based on the Hertz model, we report the elastic moduli of spray dryed cCNC microbeads. These were measured by Atomic Force Microscopy (AFM) utilizing a 1 mu m diamond-like carbon microsphere attached to the cantilever tip. This tip was shown to eliminate non-uniform readings that are otherwise obtained from sharp probe tips when interrogating rough and nanoporous surfaces. The Young's modulus and spherical morphology of cCNC microbeads were shown to depend on the spray drying parameters within a low Peclet number regime. Spray drying from dilute cCNC suspensions yielded particles with a modulus of 18 MPa. Higher cCNC feed concentrations yielded denser nanorod packing into spheres with elastic moduli on the order of 25 MPa. The microbead moduli could be chemically tuned by reacting the nanorods with a naturally sourced polyacid in the aqueous aerosol phase, without addition of a catalyst. Accordingly, citric acid additions to the nanorod feed suspensions resulted in nanorod esterification and crosslinking, whilst still yielding microspheres. Esterification increased the hybrid microbead Young's modulus to 28 MPa. cCNC-derived microbeads were found to be stiffer than microbeads derived from collagen, hyaluronic acid, alginate, or dextran, but were not as stiff as urethane-acrylate crosslinked beads, or cellulose beads reconstituted from dissolved cellulose polymers by emulsion-precipitation. Elastic modulus of cCNC nanorod microspheres tuned by aerosol phase citric acid CA ester crosslinking.